Contemporary analog cellular telephone systems use frequency modulation to impress baseband voice and data information onto a carrier signal. The modulated carrier signal is then fed to a radio frequency (RF) amplifier that includes a servo loop which precisely controls the radiated RF power level, to conform to cellular operating standards.
In the typical servo loop, the output of the RF amplifier is sampled and supplied to a power level detector. The detected power level is then compared against a reference voltage which has been selected from one of several possible voltages, each of which corresponds to one of the several output power level settings specified by the pertinent cellular operating standard. For example, in North America, there are eight possible power levels for mobile transceivers and six possible output power level settings for portable transceivers. A difference amplifier then compares the detected power level and the reference voltage to provide a difference signal. The difference signal is supplied to control the gain of the RF amplifier.
Nonlinear RF amplifiers, such as Class-C amplifiers, are particularly suited for use in current analog cellular telephone systems. They can be used directly in a system using frequency modulation. Additionally, nonlinear RF amplifiers are less expensive and consume less power than their linear counterparts. Thus, the production costs are lower and the operation times for battery operated units are longer when nonlinear amplifiers are used.
One problem with present-day analog systems, however, is the limited bandwidth available at the frequencies allocated to cellular transceivers. In an effort to reduce the expected crowding, new digital modulation operating standards have been developed. These standards specify that the baseband voice is to be digitized, combined with control information, and then impressed onto an RF carrier using a so-called O/4DQPSK modulation. This modulation requires simultaneous amplitude modulation (AM) and quadrature phase-shift keying modulation (QPSK). For further details of this modulation format, see the article by David M. Hoover, "An Instrument for Testing North American Digital Cellular Radios", in the April, 1991, Hewlett-Packard Journal.
While this modulation scheme will serve a greater number of users within a given bandwidth, it poses novel design challenges. In particular, the new standards not only require precise transmission of a signal frequency and power level, but also precise transmission of its amplitude and phase as well. Conventional wisdom is that because nonlinear amplifiers introduce amplitude and phase distortion, they cannot be used directly in a digital cellular system. Precise control of the output power level of a non-linear amplifier is also more difficult because the amplifier must exhibit a linear response at each of the possible power levels, in order to preserve the signal amplitude.
While Class-A and Class-AB amplifiers may be used to obtain a linear response, as noted earlier, they are more expensive and consume more power. Additionally, linear amplifiers typically require complicated control circuits to maintain sufficient bandwidth over a wide range of operating conditions.
What is needed is a control circuit that allows a nonlinear RF amplifier, such as a Class-C amplifier, to be used for accurately transmitting the frequency, amplitude, and phase of a digitally modulated signal. This would be particularly true if the control circuit could also be adapted for precise output power level control, as is needed in cellular transmitters.